The Kingdom of the North was one of the independent kingdoms of Westeros until the War of Conquest. It was founded during the time of the First Men by Bran the Builder, the first King in the North. At its time of greatest expansion, the Kingdom of the North included all the lands north of the Neck and south of the Gift. Bear Island became part of the Kingdom of the North after King Rodrik Stark won it from the ironborn and the rule of House Hoare in, according to legend, a wrestling match.
The Kingdom of the North was the only kingdom of the First Men that successfully resisted the Andal Invasion and clung to its ways and religion. Southron kingdoms fell and some dynasties were replaced by Andal ones and others were simply absorbed despite family names remained the same - as was the case of the ancient House Gardener.
The Kingdom of the North was absorbed into the unified monarchy of House Targaryen after King Torrhen Stark swore allegiance to Aegon the Conqueror to spare his people from the fate suffered by the armies of the Kingdoms of the Rock and the Reach in the Field of Fire.
The Kingdom of the North, however, rises again as one of the consequences of the execution of Lord Eddard Stark. Eddard's heir, Robb Stark is declared King in the North by his own vassals, as well as the Riverlords sworn to House Tully. Thus, the new Kingdom of the North includes all lands historically ruled by House Stark, and the lands surrounding the Trident, ruled by House Tully since the time of Aegon. However, Robb Stark's revived Kingdom of the North lasted for only two years until the death of King Robb and his entire army at the Red Wedding.